We go to Myrtle Beach every Spring, and have done so for the last 15 years. We know the place well. :)
I'd look at Ocean Lakes, Pirateland, Lakewood, Myrtle Beach Travel Park, or Apache. Those 5 IMO, are 'bout the best it gets at Myrtle Beach. Myrtle Beach State Park is nice, too, but its location right on the flight path of Myrtle Beach Airport, makes it a bit too noisy, IMO.
JMHO, but when you go to Myrtle beach, if you aren't beachfront or at the very least within easy walking distance to the beach.....For us it really defeats the purpose of going. Thats why we don't stay at some of the places already mentioned (KOA, Willow Tree).
We really, really like Ocean Lakes better than any of the rest, if for no other reason because they allow you to pick exactly which site you want, and they guarantee you that site. On their website, they show specific notes as well as physical dimensions (length and width) of every site. Sooo, you will know exactly what you're getting for your site, and can pick a site that works best for what you want. None of the other campgrounds in Myrtle Beach offer that.
Pirateland is a close 2nd, IMO, with the shelters they provide at each site, and the lazy river they offer. We've stayed there when we couldn't get the site we wanted at Ocean Lakes. Lakewood is pretty nice, too, we've stayed there a couple times. Those three are all located just south of Myrtle Beach, right next to each other.